In order to observe celestial X-ray sources an instrument is under development to be launched in the Astronomical Netherlands Satellite (ANS) in 1974. The aim of the experiment is to measure the spectral distribution of the sources. If there is some evidence that a source should be a pulsar the instrument can be switched from the normal mode to the pulsar mode. In this latter mode the detected photons are labeled in time using an on board clock. The ANS will have a pointing mode and a slow scanning mode. The maximum observing time per orbit for one object will be about 2000 s.